More Energy Solutions

Thirty-five years ago, when I first got involved with energy efficiency and renewable energy, the mere suggestion that one might heat with electricity would be scoffed at by those of us seeking alternatives to fossil fuels.

Amory Lovins, founder of the Rocky Mountain Institute, likened using electricity for heating to “cutting butter with a chainsaw.” Electricity is a high-grade form of energy; it doesn’t make sense to use it for a low-grade need like heating, he argued. It made much more sense, we all agreed, to produce that 75-degree warmth with solar collectors or passive-solar design.

So, it’s a big surprise that I’m now arguing that electricity can be the smartest way to heat a house. And that’s what we’re doing in the farmhouse we’re rebuilding in Dummerston. I should note, here, that all of our electricity is being supplied by a solar array on our barn.

Heat pump basics

Heating with electricity makes sense if instead of using that electricity directly to produce heat — through electric-resistance strip heaters — we use a device called a heat pump. For every one unit of energy consumed (as electricity), two to three units of energy (as heat) are delivered. This makes heat pumps significantly less expensive to operate than oil or propane heating systems in terms of dollars per delivered unit of heat.

Heat pumps use electricity in a seemingly magic way, to move heat from one place to another and upgrade the temperature of that heat in the process. Heat pumps seem like magic because they can extract heat from a place that’s cold — like Vermont’s outdoor air in January, or underground — and deliver it to a place that’s a lot warmer.

Very significantly, heat pumps can be switched from heating mode to cooling mode with a flip of a switch. In the cooling mode, they work just like a standard air conditioner.

Ground-source or geothermal heat pumps rely on the ground (or groundwater) as the heat source in the heating mode (and as the heat sink for cooling), while air-source heat pumps use the outside air as the heat source and heat sink. Because temperatures underground are much warmer than the outside air in winter, the efficiency of ground-source heat pumps is typically higher than that of air-source heat pumps.

But ground-source heat pumps are really expensive. Friends in southern Vermont have spent $35,000 — or even more — to install residential-sized ground-source heat pumps. The cost is so high because of the cost of trenching or drilling wells.

By contrast, air-source heat pumps are much simpler and far less expensive. The most common types today — and what we installed at Leonard Farm — are referred to as ductless minisplit heat pumps. There is an outdoor compressor (a box about 3 feet on a side and 1 foot deep), an indoor unit (evaporator with blower) that mounts on an interior wall, and copper tubing that carries refrigerant between the two.

The typical installed cost of a ductless minisplit system is $3,000 to $5,000, though many variables affect the cost.

These air-source heat pumps are viable today, even in cold climates, because of dramatic improvements in the past few decades. Much of this innovation has been driven by Japanese companies, including Mitsubishi, Daikin, Fujitsu, and Sanyo (now part of Panasonic).

Several decades ago, air-source heat pumps only made sense in climates that rarely dropped below 30°F in the winter; today some of these systems, including ours, will function well at temperatures below zero degrees F.

Point-source heating and cooling

Ductless minisplit heat pumps are ideally suited for compact, highly energy-efficient homes. Our house has R-values greater than R-40 in the walls and R-50 in the roof, plus very tight construction. We also have a heat-recovery ventilator (HRV) for fresh air. In tight, superinsulated homes, a single space heater (point-source heating system) can work very well, because with all the insulation fairly uniform temperatures are maintained throughout the house.

With our 1,700 square-foot house, the two upstairs bedrooms may stay a little cooler than the downstairs, but we like a cooler bedroom. In a larger house or one that isn’t as well insulated, several ductless minisplit heat pumps or a ducted heat pump option might be required.

Our Mitsubishi heat pump

We installed a state-of-the-art Mitsubishi M-Series FE18NA heat pump that is rated at 21,600 Btu/hour for heating and 18,000 Btu/hour (1 1/2 tons) for cooling. Marc Rosenbaum, P.E. ran heat load calculations showing peak heating demand (assuming –5°F outside temperature) about 23,000 Btu/hour, assuming the air leakage we measured several months ago, before the house envelope was completed. If the air leakage ends up being cut in half from that measured level, the design heat load would drop to a little over 19,000 Btu/hour.

We think the FE18NA model will work fine for nearly all conditions, but we are also installing a small wood stove — the smallest model made by Jøtul — for use on exceptionally cold nights.

The indoor unit of our heat pump is about 43 inches long by 13 inches tall by 9 3/8 inches deep. It is installed high on a wall extending in from the west wall of the house, next to an open stairway to the second floor; it is controlled with a hand-held remote. The outdoor unit, installed just off a screen porch on the west side of the house, is 35 inches tall by 33 inches wide by 13 inches deep. It is located under an overhang and held off the ground by granite blocking.

ARC Mechanical from Keene, New Hampshire, did a great job with installation, and the system has now been turned on. We won’t move in until December, but it’s nice to know we have heat.

36 Comments

High-energy-performance houses/passiv houses, etc. have something of a reputation for looking ugly. In design we should balance form and function. Did you have to place the minisplit outdoor unit right there, where it looks like somebody set an old box fan out for the trash because it didn't work anymore?

before getting too enthusiastic about air-source heatpumps. And the UK Energy Trust/DECC report on observed operational performance as against results from new units in a standards based testing environment

On our outdoor unit placement
Sonny,
The porch isn't finished. Once the screen walls are up, the outdoor compressor won't stick out like a sore thumb--at least not as much. The stem wall visible in several of the photos (which will have a cap on it, raising it slightly higher), will largely block the view from inside the porch. The location worked well due to minimal snow loading from the roof.

Snow and outdoor units
Alex,
I agree that the amount of snow that will slide off the porch roof is less than the amount of snow that will slide off your main roof.

What I would be worried about is not the snow that slides off the roof -- it's the snow that falls from the sky. Here in northern Vermont, it isn't unusual to have 3 or 4 feet of snow on the ground in March. Perhaps where you live (the banana belt of Vermont) you don't have such conditions -- or perhaps you just plan to keep your snow shovel handy.

On air-to-water heat pumps
David,
Unless I'm misunderstanding your post and the blog upon which it's based, the problems are expressed about air-to-water heat pumps, not air-to-air heat pumps. The Mitsubishi minisplit heat pump we installed is of the latter type--and that seems to be what the author of the referenced blog is suggesting.

Thanks for saying it
Thanks for saying it Sonny!
You stick an ugly box off the porch and on top of that when its summer and you want to use the porch the heat pump will be running making it hotter on the porch and nosier as well. and you didn't even mention that the indoor units make your house look like a motel 6. We'll keep the lights on for ya!

Yep, it works
I've been telling people I now heat my house with a little bit
of electricity and a lot of magic fairy dust from outside. It's
not exactly intuitive how we can get any heat from outdoor air
when it's ten degrees out, but I can watch the condensation and
evaporation temperatures on the Daikin controller diagnostics
here and see it doing exactly the right thing. I can even tell
from that when it's getting choked up with ice and is about to
go into a defrost cycle.

Recent discussion about ground-source systems seem to paint
them a little less rosy these days. Even the direct-exchange
types only deliver a COP of around 3, and apparently haven't
been meeting the higher expectations everyone had of them. Add
the extra conversion step of water-loop exchange and/or unwanted
seasonal effects that a system can create in the little chunk of
earth that it interacts with, and modern sensor-controlled
air-source looks a whole lot better.

Response to Sonny Chatum and But Why
Sonny and B.W.,
This criticism of the appearance of the Mitsubishi outdoor unit must be a northern thing. If you travel south of the Mason Dixon line, you'll see that every house has a big honking air conditioner on a concrete pad in the back yard or side yard. They are all bigger than the Mitsubishi unit installed by Alex, and everyone has them.

I guess if you live in a hot climate, you stop noticing them. They don't look abnormal after a while. (I suppose if someone from Sicily moved to Scotland, all of the chimneys would look ugly. But after a while, you get used to them.)

Aesthetics
Martin,
I, too, am from the South and I do have a big, UGLY, loud heat pump in my backyard. It's just that we have been watching Alex put up a really beautiful house with nice porch framing, the reclaimed wood from pallets on the porch and even beautiful granite blocks under the porch and then the minisplit goes up against that porch? I understand the need for it and why it is in that location, I just wish it was somewhere against the house in a less prominent location (though we really don't have an overview of where it is in relationship to the whole house and yard).

You know, the best looking outdoor minisplit I've seen was the one that Hobbit put in. He built a little roof to go on top of his. It was kind of cute.

I suggested in a previous QA question that we need to get more women concerned about energy efficiency rather than focusing the aesthetics of the home. So I know this sounds like I'm voting for looks over efficiency. I know it makes sense to use minisplits and we may end up using them in the house we are building, but I still want them to look better.

I have a big ugly heatpump
I have a big ugly heatpump three times the size of the one shown.....but it isn't up against my deck where it will blow out hot air all summer and make constant noise. I don't even have to see mine except when I cut grass. A toilet in the living room would be closer to every room in the house and you could watch TV too but I'm not going to put one there.

We just had our FE18
We just had our FE18 installed a couple months ago. So far our electric bill has barely changed from summer, so I'm really curious to see how it does this winter. It's actually undersized for our 900sf, poorly insulated house, but I will be adding insulation to get it to the point where it is right-sized and eventually oversized. My main complaint is that I wish auto mode worked better -- it seems to cool when it obviously should not, or vice versa. Only a problem in the shoulder seasons though. Other than that, the unit is extremely quiet and a fantastic piece of technology for a reasonable price.

On noise from the compressor
But Why,
I thought a lot about noise in figuring a location where the compressor could go. My wife and I certainly don't want to be listening to the compressor when we're on the porch. But here in Vermont (even the nearly tropical Banana Belt region of the state) we plan to use air conditioning very rarely. I don't expect that the heat pump will ever be operating when we're outside on the porch. And with such a well-insulated house, I expect the heat pump will only be operating in the winter when it's cold enough that we'll want to be indoors, not on the porch. If we were in Virginia where A/C would be operated all summer, we would have looked hard for another location for the compressor.

As for the aesthetics, I think it will stand out less once there are screen or screen/storm panels extending up from the stem wall behind the compressor. This isn't a deck; remember, it's a screened-in porch.

Oh man... (It's seriously gonna snow someday, Alex)
Even in central MA that compressor isn't high enough to be protected from a drifting NorEaster. In the past 15 years in Worcester we've seen seasonal snowpacks averaging 30" or more, and drifting snowstorms where the snow depths exceeded 4'. The minimum reasonable base height for a heat pump compressor in my neighborhood is 3', and even then there may be times when it has to be dug out a bit, just not EVERY FRICKIN' STORM. Bracket mounting on an exterior wall protected by the overhanging rake of the roof tends to work the best- no cornice-fall/roof avalanche adding to the depth.

In this installation there is all but guaranteed to have a curling cornice coming off the tin roof as it glides, plopping chunks on or in front of the unit. At the very least retrofit some snow-anchors onto the roof to keep the roof snow in place. Be prepared to dig it up several times this winter. On my daily commute I drive by a house in Sudbury MA (about half to 2/3 the annual snowfall compared to central MA or southern VT) heated with a pair of FE12s or FE09s mounted about as high as yours on my daily commute. The guy is good with snow shovel, and needs to be, keeping the trenches around the compressors deep enough and open enough after every storm to keep on working, and he doesn't have the roof-slide/cornice buildup issue.

This FE18 in Cambridge NY has a chance, though it would have been better to put it under the rake as opposed to the eaves:

But it has enough roof area to build up real avalanche piles under the eaves on a big snow year.

Compressor noise isn't a problem with inverter drive mini-splits- they all use scroll compressors (not the rattle of your reciprocating compressor in a window AC or refrigerator), and even at mid-speed on the blower the low 60hertz hum of the transformers on an FE18 is louder the than the whoosh of the DC driven fan. I have a relative in WA jeating & cooling with the FE18 parked just outside the living room window- can't hear it inside at all, and even standing next to it it's hard to tell if it's running unless you're standing directly in the breeze of the fan itselve, unless you force the thing into max-speed, when it sounds like white noise above the sound of the wind. On a friend's rental property in central MA he has three of them mounted about 40" off the ground next to the first-floor bedroom windows, and even standing on that side of the house you can't hear them over the sound of the highway 2000' away.

Potential issues with moving to heatpumps
As I see it there are several potential issues with moving to heatpumps for space heating.

a/ The operational COP particularly as the unit ages (or if there were installation errors) as against the COP measured in a standard test. The authoritative data that has been published indicates operational COPs can be less than what might have been hoped for.

b/ The standard tested COPs generally drop off with decreasing temperature albeit perhaps less with current models than earlier models. And, of course, at lower temps is when the unit might be expected to be operational for longer periods. If any auxiliary heating is required at low temperatures DECC argued that this should be included in the overall system measurement boundary. They called this system efficiency.

c/ In a winter peaking grid adding extra heatpump load will exacerbate the problem of meeting the current demand. A rule of thumb used locally is about $7k of generation and transmission capacity for an AC unit. This cost is currently shared amongst all grid users but this may change in future. Locally, power customers with consumption above a certain level pay part of their bill on their peak demand. This approach has been suggested for residential customers to more fairly deal with issues like AC demand.

Easy addition to deal with snow
Martin, your pictured house should have the bumpout roofline continue over the compressor. Snow problem solved, low cost, quick, and not a bad look along with no compressor performance issues.

Dana's picture looks like a very nice project but that compressor should have been put on a gable end wall up high like it is. Still, nice looking project.

Metal roofs that are allowed to slide off snow.... not my kind of roof..... Lived in one once.... flooded cellar every spring along with having to deal with the snow pack for a few weeks after all the rest of the snow was gone. With snow stops I do like metal roofs.

Response for David Coote
Winter peaking grids are often so because of the use of electric resistance heat for space heating and hot water. Utilities with winter peaking grids (such as Bonneville Power Authority) have had good success so far in moving a portion of their customers' thermal demands from electric resistance heat to heat pumps.

Regarding operational COP - the data I've seen for variable capacity "mini-split" heat pumps (as Alex noted he installed) indicates their operational COP should be a bit BETTER than their rated COP. Most heat pumps spend the majority of the heating season in part-load conditions. The variable capacity mini-splits have significantly higher COPs under part-load condition, compared to rated condition.

Average COP vs. temp (response to David Coote, #16)
In-situ field monitoring performed by Ecotope for the NEEA Northwest Ductless Heat Pump Project would indicate that in a Dummerston VT climate they should see a seasonal average COP of about 2.8 or maybe a bit better. The Eastern Idaho cluster of 10 unit near Idaho Falls that were field monitored averaged a COP of about 2.84 according to Table 33 on p.50 of this document:

The Ecotope testers measured improvements in efficiency with newer model years even within the same model that were attributed to improvements in control algorithms and other internal design features, and the predominance of older units was their conjecture for why the Inland Empire group tested somewhat lower average performance despite being in a slightly warmer climate. Within the program as a whole Mitsubishi held the lions share of installation numbers, with the FE12 and FE18 being high on the list. All units in the Eastern Idaho group were FE12s, according to the comment at the top of page 13 of the March 29 2013 addendum (p.133 of the PDF pagination.) The characteristics within a model family track very closely in bench testing (also done by Ecotope, documented elsewhere), so an FE18 in Dummerston VT will perform at about the same level that the FE12s did in Idaho Falls, or even slightly better due to the incremental design improvements over the past 3-4 years.

Bottom line, Alex should expect a heating season average COP of about 3, not much more not much less.

If it's spending major time in defrost mode due to sucking in blowing drifting snow or being totally buried, capacity (more than efficiency) may not be up to snuff at the tail end of colder snow storms, eh?

Unlike the referenced bit o' bloggery in David Coote's first post back in response #2 ( http://www.carboncommentary.com/2013/10/03/3272 ) these modulating inverter drive heat pumps get PHENOMENAL efficiency at part-load, and the lower efficiency when running flat- out. That's amply demonstrated in bench testing done by Ecotope as part of the same program. See Figure 5 in this document: http://www.nrel.gov/docs/fy11osti/52175.pdf But they do not have infinite turn-down ratios, an upsizing by more than 40-50% results in falling average efficiency due higher standby/startup power use. But in situations where they're upsized by 25% or so there is reason we might expect a slight uptick in efficiency over being exact-sized to the loads, due to improved part-load efficiencies.

Response to Martin Holladay at 9 and Alex
Martin,
I am south of the Mason-Dixon line and I replaced the big, honking heat pump compressor at the side of my house with a minisplit unit, and then added another minisplit unit at the other side of my house. Those big units really do dominate (recently built houses) around here, as you said. However, rarely are they installed in "living" or traffic areas, such as along side porches, as Alex has done, but rather at the sides of houses, where they are more out of the way and less noticeable.

Alex, if you are going to be doing some "hiding" of the unit anyway, I was wondering if you could build a lattice wall, or otherwise partition the porch area somewhat, so that you could disguise the unit somewhat while placing it ON the porch, in order to address the potential snow problem.

Another take
I agree with the gist of the comments about snow burying that Mitsu. At least it is white and will blend in with the snow. I moved from Mass. to Florida in large part to avoid naturally occurring frozen water - all our ice exists only in captivity!

I can't imagine awaking to a cold house in pre-dawn, post storm darkness and realizing my very next task would be to bundle up, don a shovel and dig out my heat pump.

I think the comment in re heating the porch during cooling operation fails to account for the direction of air moved by the system. Air will be pulled in behind the unit as pictured and driven away from the (to-be-screened) porch.

A conventional heat pump / central air outdoor unit would direct air up and out in all directions, with some of the air being captured and circulated under the porch roofline, to the discomfort and annoyance of users.

Fujitsu RLS2
As to cost , even at $0.15 / kwh the air source heat pump competes well with the cost of wood ($250/chord)

With solar panels, if you are a net generator, a heat pump is a good way to use that extra electricity. In my area ( Mass) the electric company will credit you for the extra generation, but will never send you a check. I put in my heat pump to soak up those extra kwh

I use Fujuitsu RLS2 (IIRC... EER ~13, HSPF ~12, SEER ~25) . Alas I've heated with wood since '84, and have a hard time preventing myself from throwing a log into the wood stove when I'm cold.. so the Fujitsu may not get used that much

As to oomph.. it's rated at 12K cooling and 16 k heating, and maintains a COP of ~3 down to -5F

If you want to track how they work as they get old.. the DOE has some info

EF is not the same as COP
An EF test includes standby and cycling startup losses, but it'd dead-easy to make 120F water in a 65F ambient with an air-to-water heat pump using R410a refrigerant with COP significantly greater than 3. But the size & cost of the heat exchangers (at both the air & water interfaces) required to do so gets to be a bit much. The actual operating EF of these units are greater than 2.5 under D.O.E. EF test conditions, but ~2.2-2.4 after cycling & standby loss. In warmer ambients they do better than the EF test numbers and in a 50F basements the as-used EF will be well under 2.

The real-world average COP of MODULATING air-to-water heating systems (eg Daikin Altherma) is substantially better then the on/off versions, and are quite comparable to mini-split system COPs when uses with low-temp radiation that can meet the design condition loads with 120F water. For very-low-temp radiation (concrete slab radiant floors) they can beat mini-split efficiency by a narrow margin.

The typical European residential hydronic heating system is set up to meet the design load with 140F water, which is not an efficient output temp for R410a systems, and cycling an on/off compressor at that temps for managing the part-load condition cuts even further into net efficiency, which is why the modulating variable-temp output of the Altherma gives it a significant edge. Using high-pressure CO2 as the refrigerant in air-to-water systems can deliver pretty good COPs at that temp (and higher), but it requires expensive 2-stage compressors to get there. (Sanyo had marketed such a home heating system in Europe a handful of years ago, but it has been discontinued under Panasonic ownership.) CO2 has become the standard refrigerant for automotive air conditioning, so there are likely to be future developments of mini-split sized output CO2 refrigerant heating systems coming, but don't hold your breath.

Heating tools
I think we need to consider heat pumps generically as heating tools.
They work in some situations very well, in others not so well.
I had someone call our radio program a couple weeks ago and told us that she installed 7 mini split Fujitsu's and was very happy. Her electric bill in the dead of winter in northern Maine was only $500!
This did displace $1000 of oil, so there is some merit I guess.
One has to wonder about investing the $20k she put into the mini splits into conservation, but
that deal was already done.

I am certain that any noise from an installation like Alex's is going to be extremely minimal.
Integrating with a PV system also seems to make using electricity in this manner a no brainer.
I suspect if there is a snow issue, someone will be moving it next spring after a winter of keeping it clear. There is a wood stove, so this is not really a major issue and is fixable, isn't it?

We are recycling a 5 year old Nyle Cold Climate water heating heat pump that was part of a home heating demo into our shop in the next week or two.
The folks who installed it told me that the down side of this particular unit was that the outside unit had a very loud fan. The good news is that our building is masonry, foam insulated and the outside unit will be away from everyone and everything.
Would it work for everyone? Absolutely not. We lucked out. (and it was cheap.)

Nyle is working on a variable speed air source unit that will heat water for space heating.
COPs are similar to mini-splits. We have one going in soon in another installation.
Will it fit all sizes, no, but that is what we do--help people make the right choice.

Ductless
I heated my building all year with the exception of 30 days with a Mitsubishi unit in Winnipeg Canada. The picture of how the outdoor unit is mounted is not right and may cause a problem with freezing ice on the outdoor coils since there needs to be room for the water to freely drain away during defrost. Also nice to have the outdoor unit sit in the sun so as to absorb heat in the winter months. 24 years of installing these units and there is no better unit!

Passive heat to help your heat pump
Others have commented on the location (snow issues likely to void warranty; creative mounting pads). But learned a lesson from an installation at my house: take advantage of passive heating for your mini-split. When you inevitably relocate your heat pump to protect from snow drifts, relocate to southern/western side of house (not in the shade). For those that can get past the "it's a zit on the home", elevated mounting on the warmer side of home: get above the snow &capture some of the passive heat bouncing off the home. 2 -10F warmer air into the HP means better efficiency on colder days. Use extra care for vibration isolation: at low speed/part load, unit vibration is at its worst and can carry into the home (my biggest lesson learned). you didn't buy the unit for the cooling, so put it in the sun.

Response to Mark Heizer
Mark,
Researchers have long disproved the idea that there is any performance advantage from placing an air conditioner condenser in the shade, or from placing a heat-pump condenser in the sun. The thin film of warm air that clings to the south side of a house on a sunny day is instantly exhausted when the condenser's fan turns on. Large volumes of exterior air are pulled through the equipment by the fan, and the outdoor air is all at the same temperature, whether it is pulled through the fan on the south side of the house or on the north side of the house.

It's almost guaranteed that at least 3 of those units (and probably more ) are ridiculously oversized for their zone loads, and she would have used less power and spent less cash up front with fewer ductless heads and judicious use of resistance heating for balance on the colder doored off rooms.

A typical older house in Maine has a heat load of about 50,000 BTU/hr at -7F or whatever the local 99% design temp is. Even seven -9RLS2 would have considerably more output than that. The biggest zones might have called for a 1-ton or 1.5 ton, with maybe the master bedroom or basement taking a single 3/4 ton, which would still be a lot of capacity, and maybe an installed cost getting on to $10K. But SEVEN of any size is not a solution- it's a problem. The Fujitsu -xxRLS2 series put out about 12000BTU heating per rated cooling ton -10F, according to third party testing. See figure 3, (p. 16 in PDF pagination):

Martin: Putting it in the sun helps some units shed defrost water better during sunny but below freezing days rather than letting refrozen defrost water build up in the pan over weeks with an ice-plugged drain, and eventually dinging up the fan blades. (I doubt that's an issue with the -FExxNA series but it is for some mini-splits.) You're dead right that it's not a passive-gain enhanced-efficiency issue- the localized heating of the outdoor air is not improved when you're moving hundreds of CFM through the thing.

Response to John Semmelhack
Been tied up this week so just getting back to the thread about residential heat pumps.

Responding to John Semmelhack's post:

From John "Winter peaking grids are often so because of the use of electric resistance heat for space heating and hot water."

I have heard about grids that peaked at night during supposedly off-peak periods when all the off-peak timers kicked in at 1am and turned on the DHW systems. This is easily fixed so one would hope that there are no longer significant grids that peak in this fashion. Re electric space heating, we start to get to the crux of some very interesting discussions about how to handle energy needs into the future.

From John "Utilities with winter peaking grids (such as Bonneville Power Authority) have had good success so far in moving a portion of their customers' thermal demands from electric resistance heat to heat pumps."

Could you point me at a reference showing quantitative measured data on the difference in grid supply from installing heat-pumps?

From John: "Regarding operational COP - the data I've seen for variable capacity "mini-split" heat pumps (as Alex noted he installed) indicates their operational COP should be a bit BETTER than their rated COP. Most heat pumps spend the majority of the heating season in part-load conditions. The variable capacity mini-splits have significantly higher COPs under part-load condition, compared to rated condition."

One of the points I raised was that what is important is operational measured data from installed sites including any auxiliary energy use when the heatpump is at low COP due to ambient temperature. COP's under part-load is another scenario. Could you point me at a reference showing quantitative measured data on operational heatpumps at customer sites under part-load?

Seriously, the high efficiency of these things at part load is well known, and quite different from single-speed heat pumps, and the whole point of the NEEA investigations was to put REAL numbers on it since the NEEA is a consortium of utilities with a real stake in the answer.

Part of the utilities' stake was how much to subsidize them for installation in homes previously heated with electricity to offset avoided cost of new power generation- they're dead-serious about getting it right.

Mini Split
We installed a Fujitsu Mini Split about 3 years ago for an addition to our home. These are available with multiple indoor units for 1 outdoor unit and we used 2, 1 for the addition and 1 for the master bedroom that was always hard to cool in the summer. The indoor units we chose (http://www.fujitsugeneral.com/duct.htm) are in the attic and the only thing visible are the grills on the ceiling and the return air grill on the wall along with individual thermostats for each unit. The outdoor unit is underneath the new elevated addition and even when running is very quiet, about 40db, which is rated as a library. We couldn't be happier with it. If I were to build a new house this is all I would use with individual units and thermostats for most rooms.

Maximizing TOU Rates
I have a Fujitsu 12RLS1 and a Sedore wood stove, heating 800 sqft downstairs, that can be shut off from the same area, upstairs. My winter TOU Rates for only the power portion, are almost double between 7 - 11 am and 5 - 7 pm; vs the low rate 7 pm - 7 am, (5 day rates). Ideally, I want to use more power when the outside temperature is warmest (late afternoon) and when rates are lowest. 'Bass-awkward' to the normal, night time, temperature, set back.
I think, the unit works most efficiently, with the inverter motor, working at its slowest speed. The motor seems to crank up, when the temperature is raised only 2'F Much like a former boiler-radiator heated house, it would appear best not to set back the temperature but leave it constant, even for the weekends. Occasionally, I have moved the remote from a very, slightly warmer/cooler area of the room, and the inverter motor generally 'cranks up'.
When my kids come for the weekend, I generally boost the temperature with the wood stove (using a central stairway) to the upstairs. With the kids in charge of the firewood - I am generally light! They are more willing to burn the Nuke.
Very pleased with my Mini-split; I can leave it alone! The newer model, 12RLS(2), has a heater on the outside unit and works more effectively to a lower o/s temperature.